Carbon fibers are produced by heating (carbonizing) precursor fibers made from, for example, polyacrylonitrile fibers, rayon fibers, cellulose fibers, or pitch fibers. For example, precursor fibers made from polyacrylonitrile fibers are heated (carbonized) through an oxidization process of heating the fibers in an atmosphere containing oxygen (inside an oxidization furnace), and a carbonization process of heating the fibers that have undergone the oxidization process (hereinafter, “oxidized fibers”) in an inert atmosphere (inside a carbonization furnace). Such heating (carbonization) is performed as the fibers pass (or are conveyed) through the oxidization furnace and the carbonization furnace.
To enhance the mechanical properties of fibers, appropriate tension is to be applied to the fibers during the carbonization process. In view of this, one technique uses two (or more) furnaces of different temperatures arranged in order of increasing temperatures, through which oxidized fibers pass in the carbonization process (e.g., Patent Literature 1).
Heating in the carbonization process is performed using an electric heater or the like. That is, the atmosphere inside a furnace is heated by an electric heater or the like, and oxidized fibers are indirectly heated as they pass through the inside of the heated furnace. In this case, the inside of the furnace is heated entirely, and thus the heating efficiency of the oxidized fibers is low (low heating efficiency). In addition, the heat inside the furnace is gradually conducted from an outer portion to an inner portion of the oxidized fibers (strand), and thus their carbonization takes time (prolonged carbonization).
Methods have been developed in response to such low heating efficiency and prolonged carbonization caused by the use of the electric heater or the like. One method includes directly heating oxidized fibers.
Such direct heating methods include a method using microwaves (e.g., Patent Literature 2 and Patent Literature 3), a method using plasma (e.g., Patent Literature 4), a method using both microwaves and plasma (e.g., Patent Literature 5), and a method using high-frequency electromagnetic waves (e.g., Patent Literature 6).